1. Field of the Invention
The present invention relates to a four-stroke-cycle engine for use mainly on a portable work machine and, more particularly, to a four-stroke-cycle engine equipped with an exhaust gas cleaning device which supplies secondary air to exhaust gases for re-combustion of the exhaust gases.
2. Description of Prior Art
A portable work machine such as a mower is generally mounted with a two-stroke-cycle engine as a power source which uses a mixture of fuel and lubricating oil, so that the work machine may be operated in various directions of tilt, or in all positions of operations.
In recent years, however, since it has become necessary to positively clean up exhaust gases in such a portable work machine, the mounting of a four-stroke-cycle engine having such an advantage that the exhaust gases contain little unburned components is considered. In this case, a problem lies in the weight and size of the four-stroke-cycle engine. That is, the four-stroke-cycle engine, requiring a valve mechanism as an essential component which is of complicated construction, heavy, and large in size, becomes so much heavier and larger in size than the two-stroke-cycle engine. This problem will become serious when the exhaust gas cleaning device is added to the engine. For example, it is assumed to adopt an exhaust reactor system or an oxidation catalytic system for cleaning up such unburned components in the exhaust gases as hydrocarbons and carbon monoxide. The exhaust reactor system and the oxidation catalytic system, as well known, are exhaust gas cleaning devices which supply the secondary air to an exhaust gas discharge section such as an exhaust port, and change the exhaust gases into harmless H.sub.2 O and CO.sub.2 by oxidation reaction at high temperatures. Therefore, when an exhaust gas cleaning device of the exhaust gas reactor system and the oxidation catalytic system to the engine, it becomes necessary to also use a special secondary air pump for supplying the secondary air, a gear and pulley for transmitting to the secondary air pump the torque of a crankshaft, and further a secondary air piping, which, however, will increase the size and weight of the engine. In a portable work machine whose weight must be supported by the operator, therefore, provided that the engine weight increases with the mounting of the secondary air pump, the burden on the operator also increases, resulting in a lowered handing performance.
Another problem arising in mounting the four-stroke-cycle engine on the portable work machine lies in that the lubricating oil to be applied to moving parts of the engine is likely to leak. That is, in the four-stroke-cycle engine the lubricating system includes an oil pan located below the crankcase, and in this oil pan is formed an oil reservoir. The oil pan is an essential component for the lubricating system regardless of the type of the lubricating system such as a dry sump type, a wet sump type, a forced-feed type, a splash type, etc. If a prior art four-stroke-cycle engine of known constitution is used on the portable work machine, the oil will leak from the oil pan if the portable work machine is tilted. During operation of the portable work machine, however, the machine body is often largely tilted. It is, therefore, inadequate to mount the four-stroke-cycle engine equipped with the oil pan of known construction on the portable work machine.